Thermal cyclodehydrogenations to form 6-membered rings: Cyclizations of [5]helicenes

Article abstract of DOI:10.1021/ol7015516

Flash vacuum pyrolysis (FVP) of [5]helicene induces a thermal cyclodehydrogenation to form benzo[ghi]perylene. Evidence is presented that supports an electrocyclization-rearomatization mechanism and is inconsistent with mechanistic alternatives involving the intermediacy of aryl radicals or carbenes in the helicene fjord region.

Full text of DOI:10.1021/ol7015516

ORGANIC
LETTERS
2007
Vol. 9, No. 20
3937-3940
Thermal Cyclodehydrogenations To
Form 6-Membered Rings: Cyclizations
of [5]Helicenes
Xiang Xue and Lawrence T. Scott*
Merkert Chemistry Center, Department of Chemistry, Boston College,
Chestnut Hill, Massachusetts 02467-3860
lawrence.scott@bc.edu
Received July 1, 2007
ABSTRACT
Flash vacuum pyrolysis (FVP) of [5]helicene induces a thermal cyclodehydrogenation to form benzo[ghi]perylene. Evidence is presented that
supports an electrocyclization rearomatization mechanism and is inconsistent with mechanistic alternatives involving the intermediacy of
−
aryl radicals or carbenes in the helicene fjord region.
Thermal cyclodehydrogenation reactions have been inves-
tigated since as long ago as 1872, when Graebe reported
that phenanthrene is formed by passing stilbene through a
red-hot tube (Scheme 1).1 Since that time, innumerable
Scheme 1. Gas-Phase Thermal Cyclodehydrogenations
examples of such transformations have been found.2
A
resurgence of interest in this class of reactions was sparked
in the late 1990s by the demonstration that circumtrindene
can be prepared by thermal cyclodehydrogenation of deca-
cyclene (Scheme 1),3 and other geodesic polyarenes have
subsequently been obtained by using the same strategy.4 In
2002, thermal cyclodehydrogenations played a key role in
the first chemical synthesis of C60 in isolable quantities.5
Most thermal cyclodehydrogenations require rather harsh
conditions, but some have been observed to proceed quite
readily even at relatively low temperatures in solution.6
Surprisingly little is known about the mechanism(s) of these
transformations, however, and this gap in our understanding
has stimulated us to initiate a program aimed at probing the
mechanisms of thermal cyclodehydrogenations of polycyclic
aromatic hydrocarbons (PAHs).
At the outset, it seemed unlikely to us that a single
universal mechanism would explain all thermal cyclodehy-
(1) (a) Graebe, C. Ber. Dtsch. Chem. Ges. 1874, 7, 48. (b) Graebe, C.
Liebigs Ann. Chem. 1879, 167, 161. (c) Graebe, C. Ber. Dtsch. Chem. Ges.
1904, 37, 4145.
(2) Clar, E. Polycyclic Hydrocarbons; Academic Press: New York, 1964.
(3) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996,
118, 8743.
(4) Tsefrikas, V. M.; Scott, L. T. Chem. ReV. 2006, 106, 4868.
(5) (a) Scott, L. T.; Boorum, M. M.; McMahon, B. J.; Hagen, S.; Mack,
J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500. (b) Scott,
L. T. Angew. Chem., Int. Ed. 2004, 43, 4994.
(6) Bodwell, G. J.; Bridson, J. N.; Cyranski, M. K.; Kennedy, J. W. J.;
Krygowski, T. M.; Mannion, M. R.; Miller, D. O. J. Org. Chem. 2003, 68,
2089 and references cited therein.
10.1021/ol7015516 CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/31/2007

drogenations. Therefore, several specific cases that could give
clean results were sought for study. In this regard, the closure
of [5]helicene7 (1) to benzo[ghi]perylene (2) appealed to us
as one of the simplest imaginable cyclodehydrogenations that
forms a new 6-membered ring. No competing side reactions
were anticipated.
A search of the chemical literature revealed that cyclo-
dehydrogenation of [5]helicene (1) to benzo[ghi]perylene (2)
had previously been induced both photochemically8 and by
reduction with alkali metals,9 but there seems to be no prior
report of thermal cyclodehydrogenation of 1. We therefore
synthesized a sample of [5]helicene (1) by the new olefin
metathesis route of Collins,10 subjected it to FVP, and
confirmed that it does indeed undergo a very clean and
complete conversion to benzo[ghi]perylene (2) at 1000 °C
(0.7-1.0 mmHg).
Despite the complete loss of all aromatic stabilization
energy in intermediate 5, molecular orbital calculations at
the B3LYP/6-31G(d) level of theory suggest that the elec-
trocyclization-rearomatization shown here will be energeti-
cally favored over the aryl radical and carbene mechanisms
for the thermal cyclodehydrogenation of 1.12 Intermediate 5
is calculated to lie 48.2 kcal/mol above 1 in energy. Energetic
tradeoffs that partially compensate for disruption of the
π-systems in all five benzene rings of 1 include the formation
of a new σ-bond at the expense of a π-bond and an almost
complete relief of strain energy in 5.
Superficially, the least-motions isomerization of 1 to 5
represents a conrotatory electrocyclization of a fully ben-
zannulated hexatriene and appears to violate the Woodward-
Hoffmann rules of orbital symmetry conservation.13 Inspec-
tion of the highest occupied molecular orbital (HOMO) of
1, however, reveals an in-phase overlap of the orbital lobes
at the site of bond formation (Figure 1), which makes this
pathway symmetry-allowed.
A priori, we can conceive of three plausible mechanisms
for the conversion of 1 to 2 at high temperatures in the gas
phase (Scheme 2). Aryl radical and carbene mechanisms
Scheme 2. Plausible Mechanisms for the Thermal
Cyclodehydrogenation of [5]Helicene (1)
Figure 1. HOMO of [5]helicene (1): B3LYP/6-31G(d).
To test whether or not this thermal cyclodehydrogenation
follows the electrocyclization-rearomatization pathway, we
decided to compare the behavior of 1 to that of the benzo-
[5]helicene14 6. There is no obvious reason why a remote
benzannulation should affect the strain energy of the helicene,
or the rate of the aryl radical pathway, or the rate of the
carbene pathway, and B3LYP/6-31G(d) calculations support
these intuitive conclusions.12 Thus, [5]helicenes 1 and 6
should suffer thermal cyclodehydrogenation at essentially the
same rate, if the reactions follow either the aryl radical or
the carbene mechanistic pathway.
By contrast, the rate of thermal cyclodehydrogenation by
the electrocyclization-rearomatization pathway should be
retarded significantly by the additional benzene ring, because
the aromatic stabilization energy would now be lost in six
benzene rings instead of only in five (Scheme 3, contrast
polyenes 7 and 5). Intermediate 7 is calculated to lie 53.1
kcal/mol above 6 in energy,12 which leads to the prediction
that electrocyclization of 6 should be approximately 5 kcal/
mol more difficult than electrocyclization of 1.15
analogous to the first two pathways have been studied
computationally for cyclodehydrogenations that form 5-mem-
bered rings,11 but for the formation of 6-membered rings,
the electrocyclization-rearomatization pathway must also be
considered.
(7) IUPAC name for [5]helicene (1): dibenzo[c,g]phenanthrene.
(8) (a) Tinnemans, A. H. A.; Laarhoven, W. H.; Sharafi-Ozeri, S.;
Muszkat, K. A. Recl. TraV. Chim. Pays-Bas 1975, 94, 239. (b) Grellmann,
K. H.; Hentzschel, P.; Wismontski-Knittel, T.; Fischer, E. J. Photochem.
1979, 11, 197.
(9) Ayalon, A.; Rabinovitz, M. Tetrahedron Lett. 1992, 33, 2395.
(10) Collins, S. K.; Grandbois, A.; Vachon, M. P.; Cote, J. Angew. Chem.,
Int. Ed. 2006, 45, 2923.
(12) See the Supporting Information for computational details.
(13) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Academic: New York, 1970.
(11) Violi, A. J. Phys. Chem. A 2005, 109, 7781.
(14) IUPAC name for benzo[5]helicene 6: naphtho[1,2-g]chrysene.
3938
Org. Lett., Vol. 9, No. 20, 2007

lowering the temperature could we prevent the parent[5]-
helicene from cyclodehydrogenating completely. At 850 °C,
the contrast between the two helicenes is even more pro-
nounced; 3-4 times more of the parent[5]helicene cyclizes
than does the benzannulated derivative under these conditions
(Figure 2 and Table 1).
Scheme 3
Unfortunately, no practical synthesis has been reported for
the benzannulated[5]helicene needed to conduct our experi-
ments (6).8a,16 We found it necessary, therefore, first to
develop a synthesis that could provide usable quantities of
6. Scheme 4 illustrates the route we devised. Thus, a double
Scheme 4. Synthetic Route to Benzo[5]helicene 6
Figure 2. 1H NMR spectra of the product mixtures obtained from
FVP at 850 °C of 1 (top) and 6 (bottom).
Our results are consistent with the electrocyclization-
rearomatization pathway for thermal cyclodehydrogenation
of these [5]helicenes, and they are not readily explained by
either the aryl radical pathway or the carbene pathway
outlined in Scheme 1.
One unanticipated complication was encountered in the
experiment summarized in Scheme 3, but fortunately, it does
not weaken the evidence in support of the electrocyclization-
rearomatization pathway for these thermal cyclodehydroge-
nations. In contrast to FVP of the parent[5]helicene (1),
which gives cleanly only a single product (2), FVP of the
benzannulated[5]helicene (6) gives a second, minor product
in addition to the expected hydrocarbon (8). Repeated
attempts to separate the minor product from 8 by a variety
Stille coupling between 1,2-diiodobenzene and the function-
alized 1,1′-binaphthyl 917 gives helicene 6 in good yield in
two steps from commercially available starting materials.
With ample supplies of the two helicenes available, we
then subjected both to FVP under the same conditions. As
predicted, the thermal cyclodehydrogenation of 6 was found
to be more difficult than that of 1.18 At 1000 °C, a
temperature at which the conversion of 1 to 5 is complete,
a significant portion of 6 survives FVP unchanged. Only by
Table 1. Ratios of Unchanged Helicenes:
Cyclodehydrogenation Products Following Flash Vacuum
Pyrolysis (0.5-1.0 mmHg)a
(15) Rate differences will be determined by the relative energies of the
two transition states, but for such exceedingly endothermic processes, the
relative energies of the high-energy intermediates (5 and 7) should provide
an approximate measure of the transition state energy differences (Ham-
mond’s postulate).
850 °C
1000 °C
(16) A tetrafluoro-derivative of 6 has been reported: Morrison, D. J.;
Trefz, T. K.; Piers, W. E.; McDonald, R.; Parvez, M. J. Org. Chem. 2005,
70, 5309.
[5]helicene (1)
benzo[5]helicene (6)
58:42
88:12
0:100
12:88
(17) Hoshi, T.; Shionoiri, H.; Katano, M.; Suzuki, T.; Ando, M.;
Hagiwara, H. Chem. Lett. 2002, 600.
a Determined by integration of the 1H NMR spectra of the crude pyrolysis
mixtures; error limits estimated at approximately (5%.
1
(18) For the H NMR spectrum of 8, see: Bunte, R.; Gundermann, K.
D.; Leitich, J.; Polansky, O. E.; Zander, M. Chem. Ber. 1986, 119, 3521.
Org. Lett., Vol. 9, No. 20, 2007
3939

of chromatographic methods, regrettably, all failed. In
the H NMR spectra of the hydrocarbon mixtures, both
6 relative to 1 would look even greater than it does in
Table 1. We have chosen a conservative treatment and still
find the data to be compelling. In fact, the formation of 10
as a second product from FVP of 6 actually strengthens
the evidence in favor of the electrocyclization-rearomati-
zation mechanism involving intermediate 7, as illustrated in
Scheme 3.
In sum, the thermal cyclodehydrogenation of 1 has been
found to proceed significantly faster than the corresponding
thermal cyclodehydrogenation of 6 under the same high-
temperature conditions in the gas phase. Of the three plaus-
ible mechanisms outlined in Scheme 2, only the electro-
cyclization-rearomatization mechanistic pathway readily
accounts for the experimental results, and those are in good
accord with DFT calculations. One must recognize, of course,
that no mechanism is ever “proven”; we have simply
eliminated two plausible alternatives to the electrocycliza-
tion-rearomatization pathway. In principle, other alternatives
that are consistent with our data might materialize in the
future.
1
before and after chromatography, a number of signals arising
from hydrogens of the minor product are clearly identifi-
able, including several doublets in the chemical shift
range normally associated with bay region hydrogen atoms
(8.8-9.2 ppm).19 The mass spectrum of the mixture shows
a large peak at m/z 326 (C26H14) and no evidence for a
significant quantity of C26H16, beyond the small amount
corresponding to residual starting material.
On the basis of the similarity of the minor product’s
chromatographic behavior to that of 8, the similarity of its
1H NMR signals to those of 8, and the mass spectrum of the
mixture, we tentatively concluded that the minor product was
most likely an isomer of 8 and not an isomer of 6. Following
a hunch that the minor product might have structure 10, we
synthesized an authentic sample of 1020 and were gratified
1
to see that the peaks in its H NMR spectrum match well
with the peaks of the minor product that are visible in the
spectrum of the pyrolysis product mixture.
It is not difficult to formulate a plausible mechanism
leading quite naturally to 10 from intermediate 7.21 Such a
gross skeletal isomerization would be difficult to explain,
however, without invoking intermediate 7, so we chose to
treat 10 as a second cyclodehydrogenation product. Accord-
Finally, any extrapolation of conclusions from these
experiments to systems other than [5]helicenes should be
made with caution, especially if the electrocyclization
reaction introduces significantly more strain. The mechanism
operating here is not uniVersal for all thermal cyclodehy-
drogenations that form 6-membered rings. FVP experiments
currently under investigation in our laboratory point to the
operation of a different pathway in at least one other aromatic
hydrocarbon system, and these results will be published in
due course.
Acknowledgment. The authors thank S. K. Collins
(Universite´ de Montre´al) for sharing the details of ref 10
prior to publication and A. H. Hoveyda (Boston College)
for a generous sample of olefin metathesis catalyst. This work
was supported by the Department of Energy.
ingly, the ratios of unchanged helicenes:cyclodehydrogena-
tion products reported in Table 1 were determined by adding
together the integration data for both 8 and 10 and then
comparing that sum to the integration data for unchanged
helicene 6. If 10 were treated as the result of a competing
side reaction that had nothing to do with the thermal
cyclodehydrogenation of 6, then the rate retardation seen for
Supporting Information Available: Experimental pro-
cedures and 1H NMR spectra of the crude product mixtures
from FVP of 1 and 6 at 1000 °C, XYZ coordinates and
energies calculated at the UB3LYP/6-31G* level of theory
on 1, 3-7, and the benzannulated derivatives of 3 and 4,
and proposed mechanistic pathway from 7 to 10. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(19) See the Supporting Information for copies of the 1H NMR spectra.
(20) Tinnemans, A. H. A.; Laarhoven, W. H. J. Am. Chem. Soc. 1974,
96, 4617.
(21) A mechanistic pathway from 7 to 10 that involves well-precedented
steps is proposed in the Supporting Information.
OL7015516
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Org. Lett., Vol. 9, No. 20, 2007